Xylene isomers consist of para-xylene, meta-xylene, ortho-xylene and ethylbenzene. Para-xylene is used in the manufacture of terephthalic acid which in turn is subsequently employed in the manufacture of various synthetic fibers, such as polyester. Meta-xylene is used for the manufacture of insecticides, isophthalic acid or alkyd resins. Ortho-xylene can be used as material for plasticizers.
Xylenes are found in substantial quantities in coke oven light oils and certain virgin and reformed petroleum naphthas. In the past, it has been the practice in the separation of xylene isomers either to use chemical methods or to distill meta-and para-xylene, ethylbenzene and some paraffins from a fraction containing the xylene isomers, ethylbenzene, and paraffins. Typically, para-xylene and ortho-xylene are produced by recovery of these isomers from a mixed C.sub.8 aromatic stream by means of fractionation, adsorption or crystallization. Crystallization is a separation process that takes advantage of the fact that for most xylene mixtures the melting point of para-xylene is higher than the other xylene isomers and crystallizes first. For example, para-xylene crystallizes at 13.3.degree. C., meta-xylene at -47.9.degree. C. and ortho-xylene at -25.2.degree. C.
In a para-xylene crystallizer, the C.sub.8 aromatic mixture is cooled using direct or indirect contacting means to the para-xylene crystallization temperature. The effluent from the crystallizer is a slurry comprising para-xylene crystals and mother liquor. The slurry is sent to a separation vessel, for example, a centrifuge, where the mother liquor is separated from the para-xylene crystals. Since the separation of mother liquor and para-xylene crystals is not 100% effective, the para-xylene crystals are generally remelted and recrystallized to achieve higher purity.
The purity of the melted para-xylene recovered from this slurry depends on the amount and composition of the occluded mother liquor accompanying the crystals. Mother liquor is generally present in two forms: (1) mother liquor occlusions within the crystal matrix; and (2) mother liquor that is adhering to surfaces and between crystals. The amount of mother liquor which is within the crystal matrix is dependent on the purity of the mother liquor and the method used to obtain the crystals. The amount of mother liquor which is adhering to the surface of the crystal is dependent on crystal size and shape, mother liquor viscosity and mother liquor purity. Large crystals have the favorable characteristic of containing large spaces between the crystals so that the mother liquor can drain from the crystals more readily. U.S. Pat. No. 2,913,503 describes a process for increasing the para-xylene crystal size using longer crystallization residence time. Other mechanical solutions can also be used to increase the size of the crystal.
Control of the composition of the adhering mother liquor may be accomplished by washing with a foreign substance, for example, propane at its boiling point (U.S. Pat. No. 2,823,241) or aqueous methanol (U.S. Pat. No. 2,724,007); or by washing with a stream containing a high percentage of para-xylene, for example, mother liquor from the second stage (U.S. Pat. Nos. 3,049,575 and 3,197,525).
In a conventional para-xylene crystallization process, the actual crystallization occurs in two parts or sections. The first section is a recovery section for crystallizing the bulk of the para-xylene. The second section is a purification section for enriching the recovered para-xylene. In the recovery section, the para-xylene crystals are generally formed at about -30.degree. C. to -65.degree. C. and are separated from the recovery section mother liquor using a first centrifuge at -65.degree. C. The recovered para-xylene crystals are then remelted, admixed with the recycled purification section mother liquor and passed to the purification section where high purity para-xylene crystals are formed at about 5.degree. C. to -10.degree. C. The purified para-xylene crystals are then separated from the purification section mother liquor. Mother liquor from the recovery section can be sent to a xylene isomerization zone to convert a significant amount of the non-para-xylene isomers to para-xylene which is then recycled back to the recovery section.
The problem with the conventional two-stage para-xylene crystallization process in the recovery section is very energy intensive because the entire mass C.sub.8 aromatic feed has to be cooled to the ultimate low temperatures required for effective crystallization of the para-xylene crystals, i.e. -65.degree. C.
U.S. Pat. No. 3,177,265 (issued to Lammers) discloses a process wherein in the recovery section the C.sub.8 aromatic is gradually cooled to these low temperatures by staging the recovery section temperatures using two or more crystallizers in series. In the Lammers process, a C.sub.8 aromatic hydrocarbon mixture containing 8-25 volume percent para-xylene is combined with recycled purification section mother liquor and fed to the two-stage recovery section. The recovery section comprises two crystallizers and holding tanks having a single set of recovery section centrifuges positioned downstream of the second recovery section crystallizer. In the first crystallizer the C.sub.8 aromatic mixture is cooled to -34.degree. C. to -56.degree. C., but no separation of crystals and mother liquor occurs. In the second crystallizer, the effluent from the first crystallizer is cooled to about -65.degree. C. to -76.degree. C. The separation of para-xylene crystals from its mother liquor occurs at this lower temperature. The resulting para-xylene crystals are remelted, admixed with purification section effluent, and passed to the purification section.
U.S. Pat. No. 2,866,833 (issued to Spiller) describes a para-xylene crystallization process that uses a single stage recovery section and a two-stage purification section. In the recovery section, a C.sub.8 aromatic mixture containing section. In the recovery section, a C.sub.8 aromatic mixture containing about 19 vol. % is chilled to less than -73.degree. C. to form a resulting mixture that is separated into a first solids cake containing 80 wt. % solids and a recovery section mother liquor. Purification of the first solids cake is effected in a purification section that contains two stages for crystallization and separation. In the first stage of the purification section, the first solids cake is melted, mixed in with recycled first purification stage mother liquor and chilled to about -6.degree. C. to -34.degree. C. to form a second solids cake containing about 90-95 wt. % solids and the first purification stage mother liquor. In the second purification stage, first purification stage mother liquor is chilled to about -1.degree. C. to 3.degree. C. to form a third solids cake and a second purification stage mother liquor. The third solids cake is remelted and fed back into the second stage purification stage feed. The second purification stage mother liquor is fed back to the recovery section.
Although the Lammers process may be successful at producing larger para-xylene crystals in the recovery section by staging the temperatures and increasing the residence time in the recovery section of the process, the energy consumption will be high because the entire mass of the C.sub.8 aromatic feed has to be cooled to the lowest temperature (-65.degree. C. to -75.degree. C.). Similarly, in the Spiller process, the energy consumption will remain high because the entire mass of the C.sub.8 aromatic feed must be cooled to less than -75.degree. C. Further, neither the Lammers nor Spiller process discloses or suggests rejecting the mother liquor to a xylene isomerization unit. Accordingly, a significant portion of the crystallization feed, i.e., the non-para-xylene isomers, is not converted to para-xylene. As a result, the para-xylene yield is substantially less.
There is a need for a crystallization process that: (1) recovers para-xylene crystals using less energy; and (2) increases para-xylene yield from a mixture of C.sub.8 aromatic isomers. The objective of the present invention is to address these needs.